Flexible circuits, also known as flex circuits, are widely used in applications where electrical connections are required to withstand a large number of bending cycles. A typical flex circuit contains multiple conductor traces carrying electrical signals. Conductor traces are sandwiched between two flat insulating dielectric layers. A symmetrical construction of the flexible circuit is preferred for obtaining a long flex life. The flex life of a flex circuit is the number of times the circuit can flex before failing. In a symmetrical design, the conductor traces are located on the center or neutral axis of the flexible circuit and thus encounter bending strains close to zero.
In high frequency signal applications, the close proximity of conductor traces to each other causes an electromagnetic effect known as cross-talk to occur. In cross-talk, an electric field is generated around a conductor trace which interferes with neighboring conductor traces. One solution for eliminating cross-talk is to separate the signal conductor traces with traces connected to an electrical ground. This solution is not suitable in high density applications because the number of traces is doubled, thus doubling the width of the flex circuit.
Another solution for eliminating cross-talk is by mounting a plane of conductive material connected to an electric ground in close proximity to the conductor traces. The addition of the ground plane also requires an additional layer of adhesive for bonding the ground plane. The additional layers of adhesive and the ground conductor increase the thickness of the flex circuit and move the neutral axis of the circuit away from the centerline of the conductive traces. The conductive traces are thus subjected to higher strains. This reduces the flex life of the conductor traces and the circuit. Additionally, the increased thickness of the flex circuit also reduces the flexibility of the circuit.
Accordingly, there is a continuing need for an electrically grounded flex circuit having a long flex life that is also suitable for high density applications.